The invention concerns the fusion of two digital images of an object, the first image of which favors a particular constituent of the object, while the second image favors another.
It has a particularly important application in the medical field, in which a first image of a body organ obtained by scanning is fused with a second image of the same organ obtained by magnetic resonance imaging (MRI).
In fact, an image obtained by means of a scanner particularly reveals the bony part. In such an image, the bony part is white and all the other parts, especially, the soft tissues, are of a homogeneous gray without contrast. On the other hand, an image obtained by means of MRI reveals the soft tissues in different shades of gray levels and the other parts like the bony parts and empty space are black.
In general, in the medical field a scanner image is fused with an MRI image by integrating the pixels of the bony parts of the scanner image in the MRI image.
The scanner images possess an absolute scale of gray levels, that is, all the scanner images are compatible with one another, in the sense that a given gray level always represents a particular organ. This absolute scale is the Hounsfield scale, composed of positive and negative numbers, in which the 0 level is the gray level of water.
An MRI image does not possess an absolute scale. The gray levels depend on the patient and on the image acquisition conditions. Therefore, from one MRI image to another, the muscle, for example, as soft tissue, is not represented by the same gray level. Thus, fusion of an MRI image with a scanner image results in a final image whose scale is not absolute.
In other words, the fusion of an image possessing an absolute scale with another image not possessing an absolutely scale results in a final image not possessing any absolute scale.
Furthermore, an image not possessing any absolute scale cannot be used by any of the current scanner image processing software. In fact, all of that software uses a standard gray level format, which is the Hounsfield scale.
Thus, a final image originating from the fusion of both scanner and MRI images is incompatible with any scanner image processing software. It is necessary to develop specific image processing software not calibrated on the Hounsfield scale, in order to be able to use the final image.